Electrical vapor detector



p 15', 1953 P. D. ZEMAN Y 2,652,532

ELECTRICAL VAPOR DETECTOR Filed Aug. 22. 1952 I nventor: auf D. Zemany,

Patented S ept. 15 1953 ELECTRICAL VAPOR DETECTOR Paul D. Zemany, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 22, 1952, Serial No. 305,884

' 9 Claims.

:an electrical discharge device under conditions which permit the substance to induce positive ion :formation at a more positively charged and heated electrode, the positive ions so formed being collected by a negatively charged electrode to produce an indicating current which increases with the concentration of the substance. Devices of this type have been found to be qualitatively selective at atmospheric or greater presxsures and in vacua; that is, they respond only to certain types of substances such as the al- .kali metals or other substances having ionization potentials less than the electron work function of the electrodes, the halogens or compounds 20f either. alkali metals and their compounds, the ion for- :mation apparently occurs by ionization of the substance when it comes into contact with the For certain substances such as the more positively charged and heated electrode. For others such as the halogens and their compounds, the substance appears to cause ion formation only in the presence of what might be called sensitizing materials such as the alkali metals and their compounds, and it further appears in this case that the sensitizing materials themselves are actually ionized to afford the desired positive ion current.

Since the ion currents which indicate the presence of the substance to be detected by the above-described device are often very small in magnitude, it is desirable that these currents be amplified before measurement. A principal object of my invention is, therefore, to provide an electrical vapor detector of the type described in which amplification of the small ion currents is easily obtained. Another object of my invention is to provide low signal-to-noise ratio amplification of ion currents in an electrical vapor detector. A further object of my invention is to provide an electrical vapor detector in which amplification of the ion currents is obtained with a minimum of external interference. A still further object of my invention is to provide an electrical vapor detector in which the more positively charged and heated electrode may be easily replaced.

According to one important aspect of my invention more particularly described hereinafter, there is provided apparatus for detecting the presence of a finely divided substance comprising an electrical discharge device which includes a cathode, a filament for heating the cathode to cause the emission of electrons therefrom, and an anode for receiving electrons emitted from the cathode. With proper energization of the electrical discharge device, contact of a finely divided substance with the filament induces the formation of positive ions which are collected by the cathode and which vary the potential of the cathode. The presence of the finely divided substance is detected by the changes in the electron current between the cathode and anode caused by the variations in the cathode potential.

Other objects and features of my invention will be apparent from the following description taken in conjunction with the accompanying drawings in which Fig. 1 is a, partly brokenaway and partly sectionalized view of vapor detector apparatus according to my invention; and Fig. 2 is an alternative embodiment in which identical reference characters are utilized to in dicate elements similar to those illustrated in Fig. 1.

Referring particularly now to Fig. 1, there is shown apparatus for detecting the presence of a finely divided substance according to the invention. The apparatus comprises an electrical discharge device I which includes a cathode 2, a filament 3, a control electrode 4 and an anode 5 suitably enclosed by an evacuated envelope 6. Cathode 2 is formed of a cylinder 1 which extends through envelope I as shown in order that a. finely divided substance to be detected may be introduced therewithin from an external source. The interior of cylinder 1 is isolated from the evacuated interior of envelope l by a suitable end-closure member 8. To provide support for and electrical connections to the electrodes within envelope I, various lead and support members 9, [0, ll, [2, l3 and I4 can be positioned as illustrated. The ends l5 and !6 of filament 3 can be connected to leads II and i2, respectively, by projecting them through hermetic seal members IT and I8 which are inserted in the wall of cathode cylinder 1. Electron emission from cathode 2 is obtained by means of a conventional thermionically emissive coating [9 upon the exterior surface of cylinder 1.

Circuit interconnections enabling operation of electrical discharge device I as a detector for finely dvided substances comprise an anode load resistor 20 connected in series with a current responsive instrument 2| and a source of direct voltage 22 to 'anode lead 9. Filament 3 and cathode 2 can be heated to desired temperatures by means of a transformer 23, the primary winding of which is supplied from a suitablesource of alternating current 24. Cathode cylinder 1 is maintained at a negative potential with respect to filament 3 by a source of direct voltage 25 which is connected in circuit with a resistor 26 to cathode lead I3, The secondary winding offilament transformer 23 can be maintained at ground potential as illustrated. Control electrode 4 is connected to the common interconnection of source Ziand resistor 2t througha bias voltage source 21. l

. In the operation of the apparatus'of the invention, the finely divided substance to be detected .is introduced into the interior of cathode cylinder 1, w l ,1e reby positive ion formation by filament 3. is induced. The positive ions thus formed are a collected upon the interior of the relatively negativelyeharged cathode cylinder 1 and cause a corresponding current to now through resistor 2 3 betweenfilament 3 and cathode cylinder 1.

This current fiow caused by the collection of positive ions upon cathode cylinder 1 varies the potential difference across resistor 26 and hence varies the potential of cathode 2 with respect to control. electrode 4. And since cathode cylinder .1 ,is heated to a desired temperature by filament 3 such that electron emission occurs from coating I3, the variation in the cathode-to-control electrode potential difference produces an amplified variation in theelectron current flowing in the anode circuit of the discharge device. These amplified variations in the electron current are indicated by current'responsive instrument ,2], whereby the presence of the finely divideld Substance is detected.

-While the apparatus of the invention may be employed with efficacy to detect the several finely dvided substances mentioned in the above-mentioned Ricefpatent, it is particularly adapted to the detection of the vapors of the halogens or their compounds. When the apparatus is utilizedto detect'the vapors of halogens and their compounds, filament 3 is preferably formed of a, material such as tungsten or platinum coated witha refractory coating of an oxide of one of thejmaterials disclosed andclaimed in my concurrently filed application, Serial No. 305,883. In particular, the oxides which can be employed are the oxides of aluminum, titanium, beryllium, thorium, magnesium, molybdenum,iron, calcium, manganese, silicon, cobalt, nickel and the rare earths, Even in their most purified available forms, these oxides contain at least a few hundredths of a per cent of alkali impurities which evaporate from the filament coating as alkali ions during operation of the apparatus to detect halogens. .With a thin coating (e. g., several mils in thickness) of an oxide of one of these materials upon filament 3, sufiicient current should be passed from filament transformer 23 through filament 3 to heat the coating to a temperature ranging from about 700 C. to about 1300 Cl'or higher. Atsuch temperatures, the above oxides "apparently possess semiconductor properties which allow them to serve as insulators for the direct flow of current between filament 3 4 and cathode cylinder 1 but still permit the evapo-' ration of alkali ions in the presence of halogen vapors. Therefore, when the oxides of the aboveidentified materials are employed as coatings for filament 3, it is unnecessary to take special precautions to prevent the short-circuiting of filament 3 to cathode cylinder 1. It is not essential that filament 3 .have'the twisted configuration illustrated in Fig. l, and other convenient forms or shapes can be employed providing sufiicient passages are provided within cathode cylinder 1 to permit exposure of considerable surface of the coating to the vapors of halogens or their compounds introduced into the cathode cylinder. Moreover, I have found that in some instances filament 3 may be constructed of a thin uncoated strip or cylinder of platinum or other metals which contain sufiicient alkali impurities to serve as emitters of positive ions in the presence of halogen vapors. I

It will be apparent Train the foregoing that the electrical "discharge device of the invention may be built in accordance with well-known vacuum tube construction "techniques. In'some applications whereciirrent gain is relatively unimportant, controlelectrode 4 and its associated circuit may be omitted whereupon the changes in anode current due to variations in the anodeto-cathode potential diiferencecan serveto indicate the presence of the finely divided substance. 'It will also be understood that electrodes. in addition to control electrode4,j'ma'y be used to achieve 'desired'ci'rcult characteristics; for example, tetrode, pento'de, dual triode, etc., electrode structures can be utilized. If additional amplification is desired to supplement the amplification provided by discharge device I further stages of amplificationmay be coupled to anode 5 in manners well knownto thosefamiliar with electroniube circuitry. Inthe specific circuit shown in Fig. 1, advantageous operation" can also be obtained bybdnn'e'cting the negative terminal of source 22to the end of resistor '23 opposite the end to 'Which'cathode2 isconnected.

It should beparticularly observedthat the apparatus of theinv'e'ntionca n be employed to detect a finely divided substance in an evacuated system or at atmospheric or -higher"pressures. Thus-the substance to bedetecte'd' may be circulated throughcatho'de cylinder? at atmospheric or higher pressures' or it may be permitted to diifuse intoc'athode cylinder! "from a vacuum system connected thereto. The forcing of gaseous media containing the finely divided substance to be detected through cathode cylinder 1 can-be facilitated by the removal of end-closure member 8- and the continuance of cathode cylinder 1 through envelope fi. -In'such event; envelope 3 must 'be sealed'tothe exteriorof cathode cylinder 'in'order that a'vacuum may be'maintained within the envelope. In Fig.2, 'there 'is shown a1ra1ternative"em bodiment of the"electricaldischarge device ofmy invention whichfa'cflitates ready removal-and replacement of filament 3. -As'=illustrated, ends 15 and 'l6of filament" 3 extend through cylinder 1 which is terminated in a convenient location by a sealportion 30. Ends 15 and l 6 are sealed along with leads H a'nd 'IZ'intO a press seal-3| which is attached to aseal portion 32 that'mates with portion 30. The finely divided substance can beintroduoedto'rcylinder I "through aside arm 33. Accessto'filament 3 'iseasily' obtained by merely removing *seal' 3 l and sealportion 32, from seal portion 30.

While I haveshown and described particular embodiments of my invention,it will be obvious to those skilled in the art that various changes and modifications can be made without departing from the invention, and I, therefore, aim in the appended claims to cover allsuch changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for detecting the presence of a finely divided substance comprising an electrical discharge device which includes a cathode, an electrode for heating said cathode to cause the emission of electrons therefrom and an anode for receiving electrons emitted from said cathode, means for maintaining said anode at a positive potential with respect to said cathode and said cathode at a negative potential with respect to said electrode, said cathode, said electrode and said anode being arranged such that contact of the finely divided substance with said electrode induces the formation of positive ions which are collected by said cathode, means for causing the potential of said cathode to vary in response to the collection of positive ions thereby, and means for indicating changes in the electron current between said cathode and said anode caused by the variations in said cathode potential, whereby said changes in the electron current serve to detect the presence of the finely divided substance.

2. Apparatus for detecting the presence of a finely divided substance comprising an electrical discharge device which includes a cathode, an electrode for heating said cathode to cause the emission of electrons therefrom, a control electrode and an anode for receiving electrons emitted from said cathode, means for maintaining said anode at a positive potential with respect to said cathode, said cathode at a negative potential with respect to said heating electrode and said control electrode at a desired control potential with respect to said cathode, means for bringing a quantity of the finely divided substance into contact with said heating electrode to induce the formation of positive ions which are collected by said cathode, means for causing the control electrode to cathode potential difference to vary in response to the collection of positive ions by said cathode, and means for indicating changes in the electron current between said cathode and said anode caused by the variations in said control electrode to cathode potential difference, whereby said changes in the electron current serve to detect the presence of the finely divided substance.

3. Apparatus for detecting the presence of a finely divided substance comprising an electrical discharge device which includes a cathode, an electrode for heating said cathode to cause the emission of electrons therefrom, a control electrode and an anode for receiving electrons emit-, ted from said cathode, a first circuit connected between said anode and said cathode including a load resistor and a source of direct voltage for maintaining said anode at a positive potential with respect to said cathode, a second circuit connected between said cathode and said heating electrode including a resistor and a source of direct Voltage for maintaining said cathode at a negative potential with respect to said heating electrode, means for maintaining said control electrode at a desired control potential with re- 6 spect to said cathode, means for bringing a duantity of the finely divided substance into contact with said heating electrode to induce the formation of positive ions which are collected by said cathode and cause a current to flow insaid second circuit whereby said control potential is varied by said current flow through said resistor in said second circuit, and means for indicating changes in the electron current in said first circuit caused by the variations in said control potential, whereby said changes in the electron current serve to detect the presence of the finely divided substance.

4. Apparatus as in claim 3 in which said load resistor is connected in series with said source of direct voltage in said second circuit and said means for maintaining said control electrode at a desired control potential with respect to said cathode includes a source of direct voltage connected between said control electrode and the common interconnection of said load resistor and said source of direct voltage in said second circuit.

5. An electrical discharge device for detecting the presence of a finely divided substance comprising a cathode and an anode disposed within an evacuated envelope, said cathode including a hollow electrode which extends through the wall of said envelope to provide a means for introducing a quantity of the finely divided substance into the interior of said hollow electrode, and an electrode adapted for heating to an elevated temperature and disposed within said hollow electrode for causing in the presence of the finely divided substance the formation of positive ions which may be collected upon said cathode and further for heating said cathode to an elevated temperature, whereby electrons may be emitted from said cathode and received by said anode in dependence upon the number of positive ions collected by said cathode.

6. An electrical discharge device as in claim 5 which further comprises a control electrode disposed within said envelope between said cathode and said anode.

7. An electrical discharge device as in claim 5 in which said hollow electrode is hermetically sealed to isolate its interior from the interior of said evacuated envelope.

8. Apparatus for detecting the presence of a finely divided substance comprising an electrical discharge device which includes a cathode, a control electrode, an electrode forheating said cathode to cause the emission of electrons therefrom, an anode for receiving electrons emitted from said cathode and an evacuated envelope enclosing said anode, said control electrode, said heating electrode and at least a portion of said cathode, said cathode including a hollow electrode which extends through the Wall of said envelope, means for maintaining said anode at a positive potential with respect to said cathode, said cathode at a negative potential with respect to said heating electrode and said control electrode at a desired control potential with respect to said cathode, means for introducing a quantity of the finely divided substance into said hollow electrode to induce the formation of positive ions which are collected by said cathode, means for causing the control electrode to cathode potential difference to vary in response to the collection of positive ions by said cathode, and means for indicating changes in the electron current between said cathode and said anode caused by the variations in said control electrode to cathode potential difference, whereby said changes in the 9. Apparatus as m laiirl 8 in which said hol- 10w electrode is hrmetically e'ald t0 isolate its interior from the intfidl bf said evaGflatd en-' velope.

PAULi D. ZEMANY.

Number 1 35%? 2x55954 5 2,625,586

UNITED STATES PATENTS 

